The present invention relates to an automatic exposure control system for copier systems which utilize flash exposure of a document. More particularly, it relates to a control and compensation circuit which senses the amount of radiant energy reflected from the object onto walls of an optical housing while exposure of a photosensitive material is occurring and terminates the flash after sufficient exposure level is reached.
When an object is reproduced, either photographically or electrophotographically, by flash exposure of a light sensitive material, the density of the object (dark and light highlights) must be determined in some manner so that compensation can be made to maintain exposure uniformity. Prior art efforts to control exposure automatically are reflected in numerous publications. In photographic applications, an automatic exposure control for an electronic flash unit typically includes a light meter and electronic circuitry which detects the instantaneous amount of light reflected from the scene to be photographed onto the light sensitive element of the light meter. The light meter contains circuitry which integrates the signal derived from the light detecting element. When the integrated signals enter a predetermined level corresponding to required film exposure the flash is extinguished. Representative of various such arrangements are U.S. Pat. No. Re. 28,783; U.S. Pat. Nos. 3,350,604; 3,756,132; 3,776,112; 3,783,336 and 4,132,925.
In addition, various exposure control circuits are known in the photographic art which vary reference voltage with respect to time. The most relevant of these are disclosed in U.S. Pat. Nos. 3,737,721; 3,774,072; 3,985,440 and 4,101,812.
These photographic systems operate over relatively large dynamic ranges (compared to copiers) and the exposure control circuits are typically designed to tolerate errors upwards of 25% (1/4 f/stop). An exposure control system, in order to obtain uniform exposure of a photoreceptor, must be considerably more accurate; in the order of .+-.6% error, or better. Some representative automatic exposure control systems in a copying environment are disclosed in U.S. Pat. Nos. 3,985,440; 3,998,547; 4,017,180; 4,093,376 and 3,947,117. While these control systems reduce exposure errors considerably, they either require complex circuitry to implement or do not reduce exposure errors to the small tolerances required to produce increasingly upgraded copy quality.